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I'm making a calciner that heats gypsum to a heat of 350 degree Celsius. A burner will be used to heat the rotary kiln to a constant temperature of 350 degree Celsius. I'm looking for the ideal insulation material to make the housing of the keep the heat onto the kiln inside the calsiner. The material must not be to heavy. I'm looking for a material something like fireclay, but something more ideal to build a housing of. The insulation will be put inside the housing like in the image.

is showing numbers like 2300 F (1260 C) and 2600 F (1427 C) in the titles of these product offerings. But unfortunately the descriptions for these do not seem to have any numbers for thermal conductivity.

Whenever possible I use rockwool for these tasks.Light and very temp proof to around 800°C if the oxygen levels are normal and much higher with little to no oxygen present.Similar story for housings, here I prefer cement sheeting as it is hard, durable and can tolerate medium heat.At below 600° C you can use aluminim piping as used for solid air ducts or galvanised steel ducts/ downpipes.If you use two very different diameters you could even fill the void with rockwool and compress it a bit for stability until you permanently mount all.